Quantum dot light emitting diodes for multiplex gas sensing

ABSTRACT

A gas detection device comprising a light emitting source including a first plurality of quantum dots of substantially discrete size and made of a semiconductor material a gas cell to contain the gas to be detected and a light detector.

CROSS REFERENCE TO RELATED APPLICATIONS/PRIORITY

The present invention claims priority to United States ProvisionalPatent Application No. 62/126,495, filed on Feb. 28, 2015, which isincorporated by reference into the present disclosure as if fullyrestated herein. To the extent that there is any conflict between theincorporated material and the present disclosure, the present disclosurewill control.

FIELD OF THE INVENTION

The present invention relates to the field of quantum dot light emittingdiodes for gas sensing, detection, and identification.

BACKGROUND OF THE INVENTION

The detection and quantification of gas, especially the flammable andpoisonous gases in air are extremely important due to their greatdemands in areas such as environmental monitoring, coal mine safety,industrial production, and automobile exhaust monitoring, etc. A numberof methods have been used to detect gas species including spectroscopy,electrochemistry, and photoacoustics, each having various disadvantages.Multiple gases often co-exist in complicated environments, furthercomplicating design successful of detection devices. For example, thegas in coal mines may compose methane (CH₄), ammonia (NH₃), carbonmonoxide (CO), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), etc. CH₄ isunavoidable to be released during the coal mining and it outbursts inboth underground and surface mines, but must be tracked and monitored.When CH₄ reaches a concentration of 5-15% in air in a closedenvironment, CH₄ becomes dangerously explosive. Similarly, C₂H₂ and NH₃are released during industrial production and they are extremelyexplosive gases when their concentrations reach 2.5%-80% and 16-25%,respectively. Therefore, multiplex gas detection is quite necessary, andusually several wavelengths are required for the simultaneous multiplexgas detection using spectroscopy method. Traditional instrument's lightsources usually generate single wavelength light, and their physicalsize is bulky, their optical stability and specificity are low.Additionally, simultaneous detection and measurement of multiple gasesis difficult.

SUMMARY OF THE INVENTION

Wherefore, it is an object of the present invention to overcome theabove mentioned shortcomings and drawbacks associated with the priorart.

The inventor has observed that semiconductor quantum dots (QDs) usuallypossess high photoluminescence (PL) quantum yield (QY) with sizedependent tunable wavelength emission that make them promising, lightemitting diodes (LEDs) as light sources, which can be used for theoptical detection and measurement of gases.

The present invention relates to making multi-wavelength light emittingdiodes (LEDs) without mutual-absorption using QDs. The LEDs emitmultiple emission wavelengths for detecting multiple gasessimultaneously through direct absorption spectroscopy.

The present invention relates to a gas detection device and methodincluding a light emitting source including a first plurality of quantumdots of substantially discrete size and made of one or moresemiconductor materials, a gas cell to contain the gas to be detected,and, light detector.

Further optional embodiments include a convex mirror arrangement toincrease a light path through the gas cell to multiple times a length ofthe gas cell.

Further optional embodiments include a first layer of quantum dotsincluding the first plurality of quantum dots, a second layer of quantumdots includes a second plurality of quantum dots, and the firstplurality of quantum dots being one of substantially different size,substantially different composition, and both substantially differentsize and substantially different composition compared to secondplurality of quantum dots.

Further optional embodiments include a third layer of quantum dotsincluding a third plurality of quantum dots, and the third plurality ofquantum dots being one of substantially different size, substantiallydifferent composition, and both substantially different size andsubstantially different composition compared to each of the first andthe second plurality of quantum dots.

Further optional embodiments include the first and/or second and/orthird plurality of quantum dots including at least one of Pb, Se, S, Te,Zn, Cd, Cu, In, P, and a combination thereof.

Further optional embodiments include first and/or second and/or thirdplurality of quantum dots including at least one of PbSe, PbS, PbTe,ZnS, ZnSe, CdSe, CdTe, CdS, CuInS₂, and InP.

Further, optional embodiments include first and/or second and/or thirdplurality of quantum dots are between 4 nm and 7 nm in size.

Further optional embodiments include the first and/or second and/orthird plurality of quantum dots exhibit photoluminescence with awavelength emission in at least one of Infra-red, Near Infra-Red,visible, and ultraviolet wavelengths of light.

Further optional embodiments include the light emitting source includinga multi-wavelength light emitting diode.

Further optional embodiments include the multi-wavelength light emittingdiode not exhibiting mutual-absorption.

Further optional embodiments include the first and/or second and/orthird plurality of quantum dot includes a material with a quantum yieldof at least 85%.

The present invention further relates to a device for and method ofdetecting one or more gasses using a light, emitting source having atleast a first plurality of semiconductor quantum dots of substantiallydiscrete size, a gas cell to contain the gas to be detected and a lightdetector. The method comprises the steps of energizing the lightemitting source, emitting first light waves from the first plurality ofquantum dots, and passing the first light waves through a gas sample inthe gas cell and into a light detector.

Further optional embodiments include the light emitting source includinga second plurality of semiconductor quantum dots of substantiallydiscrete size and further comprising the step of emitting second lightwaves from the second plurality of quantum dots and passing the secondlight waves through the gas sample in the gas cell and into a lightdetector.

Further optional embodiments include the light emitting source includinga third plurality of semiconductor quantum dots of substantiallydiscrete size and further comprising the step of emitting third lightwaves from the third plurality of quantum dots, passing the third lightwaves through the gas sample in the gas cell and into a light detector,and the first, second, and third light waves being emitted atsubstantially a same first time and the first, second, and third lightwaves are detected at substantially a same second time.

Further optional embodiments include the gas sample containing one of asingle gas, more than one gas, and at least three gasses.

Further optional embodiments include one of detecting at least one gasand detecting two or more gases.

Further optional embodiments include reflecting the first light wavesoff of mirrors and passing the first light waves through the gas samplemore than once before passing the first light waves into a lightdetector.

Further optional embodiments include first layer being adjacent to oneof a blue or ultraviolet light emitting diode chip; the first pluralityof quantum dots having a first wavelength photoluminescence emission;the second layer being adjacent to the first layer and the secondplurality of quantum dots having a second wavelength photoluminescenceemission that is shorter than the first wavelength, and the third layerbeing adjacent to the second layer and spaced from the first layer bythe second layer, and the third plurality of quantum dots having a thirdwavelength photoluminescence emission that is shorter than the first andthe second wavelength.

The present invention additionally relates to a gas detection device andmethod comprising a non-mutually absorption multi-wavelength lightemitting diode; the light emitting source including a first plurality ofquantum dots of substantially discrete size and made of a semiconductormaterial, a second plurality of quantum dots of substantially discretesize and made of a semiconductor material, and a third plurality ofquantum dots of substantially discrete size and made of a semiconductormaterial; the first plurality of quantum dots are one of substantiallydifferent size, substantially different composition, and bothsubstantially different size and substantially different compositioncompared to second and the third plurality of quantum dots; the secondplurality of quantum dots are one of substantially different size,substantially different composition, and both substantially differentsize and substantially different composition compared to third pluralityof quantum dots; the light emitting source including at least threelayers, the first layer including the first plurality of quantum dotsand not the second or third pluralities of quantum dots, the secondlayer including the second plurality of quantum dots and not the firstor third pluralities of quantum dots, and the third layer including thethird plurality of quantum dots and not the first or section pluralitiesof quantum dots; a gas cell to contain the gas to be detected; the cellincluding a convex mirror arrangement to increase a light path throughthe gas cell to multiple times a length of the gas cell; a lightdetector; one of the first, the second, and the third pluralities ofquantum dots includes at least one of PbSe, PbS, PbTe, ZnS, ZnSe, CdSe,CdTe, CdS, CuInS₂, and InP; the first, the second, and the thirdpluralities of quantum dots are between 4 nm and 7 nm in size; and thefirst, the second, and the third pluralities of quantum dots eachexhibit photoluminescence with a wavelength emission in at least one ofInfra-red, Near Infra-Red, visible, and ultraviolet wavelengths oflight.

Various objects, features, aspects, and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the invention, along with theaccompanying drawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate various embodiments of theinvention and together with the general description of the inventiongiven above and the detailed description of the drawings given below,serve to explain the principles of the invention. The invention will nowbe described, by way of example, with reference to the accompanyingdrawings in which:

FIG. 1 is a normalized absorption spectrum and Transmission ElectronMicroscopy (TEM) image of 4.6 nm PbSe QDs;

FIG. 2 is a normalized absorption spectrum and TEM image of 5.1 nm PbSeQDs;

FIG. 3 is a normalized absorption spectrum and TEM image of 6.1 nm PbSeQDs.

FIG. 4 is a normalized PL spectrum of 4.6 nm PbSe QDs and the coveredabsorption line intensities of C₂H₂ gas;

FIG. 5 is a normalized PL spectrum of 5.1 nm PbSe QDs and the coveredabsorption line intensities of CH₄ gas;

FIG. 6 is a normalized PL spectrum of 6.1 nm PbSe QDs and the coveredabsorption line intensities of NH₃ gas;

FIG. 7 is a schematic representation of the fabrication of near infrared(NIR) QD-LED with PbSe QDs;

FIG. 8 is a variation of LED spectrum fabricated by different ways: (1)5.1 nm PbSe QDs were deposited firstly followed by 6.1 nm PbSe QDs; (2)6.1 nm PbSe QDs were deposited firstly followed by 5.1 nm PbSe QDs; and(3) 5.1 and 6.1 nm PbSe QDs were mixed and deposited together;

FIG. 9 is an evolution of a spectrum of a QD-LED, fabricated accordingto the method shown in FIG. 7; at different working bias from 2.5 to3.5V

FIG. 10 is a schematic illustration of the detection setup;

FIG. 11 is a PL spectrum of the LED after absorbed by a certain amountof single gas C₂H₂;

FIG. 12 is a PL spectrum of the LED after absorbed by a certain amountof single gas CH₄;

FIG. 13 is a PL spectrum of the LED after absorbed by a certain amountof single gas NH₃;

FIG. 14 is a variation tendency of optical power (area integral) of theother two PL peaks after absorbed by C₂H₂.

FIG. 15 is a variation tendency of optical power (area integral) of theother two PL peaks after absorbed by CH₄;

FIG. 16 is a variation tendency of optical power area integral) of theother two PL peaks after absorbed by NH₃;

FIG. 17 is an area integral of PL spectrum of C₂H₂ at 15 standardconcentrations and the working curve fitted by Matlab, with the insetshowing the accuracy curve;

FIG. 18 is an area integral of PL spectrum of CH₄ at 15 standardconcentrations and the working curve fitted by Matlab; with the insetshowing the accuracy curve;

FIG. 19 is an area integral of PL spectrum of NH₃ at 15 standardconcentrations and the working curve fitted by Matlab, with the insetshowing the accuracy curve;

FIG. 20 is an evolution of LED spectrum with the concentration change ofmixed gases;

FIG. 21 is a comparison between standard and measured concentrations ofC₂H₂ from FIG. 20;

FIG. 22 is a comparison between standard and measured concentrations ofCH₄ from FIG. 20; and

FIG. 23 is a comparison between standard and measured concentrations ofNH₃ from FIG. 20.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be understood by reference to the followingdetailed description, which should be read in conjunction with theappended drawings. It is to be appreciated that the following detaileddescription of various embodiments is by way of example only and is notmeant to limit, in any way, the scope of the present invention.

This patent application describes a design and fabrication ofnon-mutual-absorption QD-LEDs with multiple emitting wavelengths usingQDs and an excitation light source. The excitation light source having alight wavelength within the QD's absorption range, for example, a blueor ultraviolet GaN LED. The disclosed method and apparatus shows goodselectivity of multiplex gas simultaneous concentration measurementindicating that the disclosed QD-LED light sources have great potentialin many fields on account of their low-cost, small size, highefficiency, and multiplex detection. Near infrared (NIR) QD-LEDs aredescribed in detail because many simple but flammable gases have strongabsorption in NIR range. The NIR QD-LED design is also good for visibleQD-LEDs and the gas detection with visible range absorption.

PbSe bulk material has a small band gap of 0.28 eV at room temperatureand a very large exciton Bohr radius of 46 nm. As a result, PbSe QDsshow very strong quantum confinement and high quantum yield (QY). ThePbSe QDs' QY can easily reach 85% or higher in NIR region. PbSe QDs'band edge absorption peaks span over a wide infrared wavelength regionof 1-4 μm. An LED emitting 3 NIR wavelengths using PbSe QDs is describedas an example. The as-fabricated NIR LEDs are then applied to detectthree gases (NH₃, CH₄ and C₂H₂) at the same time based on directabsorption spectroscopy. These three gasses are just an example of thedetection abilities of the gas detector using QD-LEDs.

The PbSe QDs employed in the present study were synthesized according tothe method reported by Yu et al. (Yu, W. W.; Falkner, J. C.; Shih, B.S.; and Colvin, V. L.) Preparation and characterization of monodispersePbSe semiconductor nanocrystals in a noncoordinating solvent. Chem.Mater. 2004, 16, 3318-3322.). The method is described in the article isincorporated herein by reference. Briefly, a mixture of 0.892 g PhO,2.26 g oleic acid (OA), and 12.848 g 1-octadecene (ODE) were loaded intoa 100 mL three-neck flask. After 10 minutes nitrogen flow to remove theoxygen, the three-neck flask was heated to 170° C. After PbO powdercompletely disappeared and the solution became colorless, 6.9 mLSe-trioctylphosphine solution (containing 0.637 g Se powder) was quicklyinjected into the vigorously stirred solution. The temperature of thereaction mixture was then maintained at 143° C. for QD growth. At acertain reaction time, 30 mL of toluene was injected into the three-neckflask and then the flask was submerged in a room-temperature water bathto completely quench the reaction. A series of purification operationprocedures were carried out to remove excess reaction precursors and ODEbefore utilization. QDs were purified by phase extraction twice withmethanol and precipitation once with acetone. The final products weredispersed in chloroform for LED fabrication and stored in an argonfilled container.

UV-Vis absorption spectra were recorded using a Shimadzu UV-3600UV-visible spectrophotometer. The photoluminescence properties of PbSeQDs in chloroform solutions and the spectra properties of LED weremeasured on an Omni-λ300 Monochromotor/Spectrograph. All the absorptionand PL spectra were recorded at room temperature, or betweenapproximately 20 and 23° C. A JEOL FasTEM-2010 transmission electronmicroscope (TEM) was used for observing the particle size and shape. TheTEM specimens were prepared in a glove box, where purified PbSc QDs weredispersed in chloroform and dropped on carbon-coated copper grids, andthen the solvent was evaporated off.

Three sets of PbSe QDs with respective particle sizes of 4.6, 5.1, and6.1 nm were chosen. Their respective first absorption peaks of 1437,1592, and 1862 nm are shown in FIGS. 1-3. FIG. 4 shows the PL spectrumof 4.6 nm PbSe QDs and the absorption spectrum of C₂H₂. The PL peak of4.6 nm PbSe QDs is 1515 nm with a full width at half maximum (FWHM) of150 nm which covers the entire absorption spectrum of C₂H₂ gas (from1500 to 1550 nm). The PL spectrum of 5.1 nm PbSe QDs has a PL peak of1665 nm and a FWHM of 143 nm which, as shown in FIG. 5, corresponds tothe CH₄ absorption spectrum. FIG. 6 indicates the PL peak of 6.1 nm PbSeQDs locates on 1943 nm with a FWHM of 185 nm; the main absorptionspectrum of NH₃ from 1900 nm to 2060 nm is completely covered by thisemission light. The highest absorption coefficients of CH₄, C₂H₂ and NH₃are 1.33×10⁻²¹, 1.34×10⁻²⁰ and 1.22×10⁻²⁰ cm⁻¹.

An NIR LED based on PbSe QDs with three particle sizes was fabricated asshown in FIG. 7. A blue (or ultraviolet) GaN LED was employed as theexcitation light source and PbSe QDs were used as photoluminescentmaterials. In step S1 PbSe QDs were mixed with chloroform. In step S2the PbSe QD/chloroform mixture was mixed with a UV glue. In step S3 thePbSe QD/chloroform/UV glue mixture as mixed by vortex and ultrasonictreatment to form a homogeneous mixture. In step S4 the mixture wastransferred into a vacuum chamber to remove chloroform and bubbles. Instep S5 the PbSe QDs/UV glue composites were applied on the GaN chip toform a layer. In step S6, steps S1-S5 were repeated two times. The firstrepeat resulted in a second sized PbSe QDs-UV glue composite beingapplied on top of the first layer to form a second layer. The secondrepeat resulted in a third sized PbSe QDs-UV glue composite beingapplied on top of the second layer to form a third layer. Wherefore,three thin layers with three (3) sizes of PbSe QDs were formed.

According to FIGS. 1-6, the emission from QDs with small particle sizewill be absorbed by QDs with large particle size. If all the PbSe QDswith three particle sizes were mixed together, the device performancewould be limited because of the reabsorption among different size QDnanoparticles, which will affect the output intensity of the LED device.FIG. 8 shows the different LED output intensity when using two sized QDnanoparticles with different layer formation as an example. Three LEDswere fabricated with 5.1 nm and 6.1 nm PbSe QDs (the concentration ofthe two QDs in UV glue was 0.5×10⁻³ mmol·L⁻¹). 5.1 nm and 6.1 nm QDs-UVglue composites were deposited on the LED chip one after another whichwas named LED 1. Their thicknesses were adjusted to be 165.5 μm and 48μm. Similarly, the deposition order changed and the thicknesses werekept the same then LED 2 was obtained. 5.1 nm and 6.1 nm QDs-UV gluecomposites were mixed at a 165.5:48 volume ratio, and then the mixturewas deposited on the chip to form a film of 213.5 μm thick to get LED 3.After ultraviolet light curing, the LEDs' spectra were recorded as shownin FIG. 8. Compared with LED 2, the intensity of the emission spectrumof 5.1 nm QDs in LED 1 was stronger because much more blue light wasabsorbed by 5.1 nm QDs in LED 2. Due to the less blue light irradiatingand the reflection of 5.1 nm QDs in LED 1, the intensity of emissionspectrum of 6.1 nm QDs decreased seriously despite absorbing the lightemitted by 5.1 nm QDs. Also, the emission spectrum intensities of 5.1 nmand 6.1 nm QDs in LED 3 decreased simultaneously compared with LED 2.After being mixed with 6.1 nm QDs, the light emitted by 5.1 nm QDs wasabsorbed significantly by 6.1 nm QDs. LED 2 demonstrated the wellbalanced emission which is the best.

This substantially no mutual-absorption layered structure proved to bean effective way to optimize the output intensity as it weakens thereabsorption between the QDs. Therefore, 6.1 nm PbSe QDs were firstlydeposited on the GaN chip followed by 5.1 nm and 4.6 nm PbSe QDs tofabricate an NIR QD-LED with strong multiple emitting wavelengths. Theconcentration of PbSe QDs with different particle sizes in UV glue wasat or about 5×10⁻³ mmol·L⁻¹. QD's luminous intensity could be controlledby adjusting the PbSe QDs-UV glue composites thickness in this design.The thicknesses were determined to be 48.0, 165.5, and 671.5 μm for 6.1,5.1, and 4.6 nm PbSe QDs, respectively.

The luminescence spectra of the as-fabricated NIR QD-LED under differentforward bias are shown in FIG. 9, in which the emission peaks of PbSeQDs were located at 1526, 1676, and 1949 nm, respectively. When theforward bias increased from 2.8 V to 3.2 V, the intensity of threeemission peaks increased simultaneously with a good stability.

EXEMPLARY QD-LED GAS DETECTION DEVICE. FIG. 10 shows an exemplary gasdetecting device 2. This experimental setup consisting of the NIR QD-LEDlight source 4, the gas cell 6 and the optical spectrometer 8. Impulsevoltage was employed to drive the light source 4. After being convergedby the convex lens 10, an NIR beam 12 was transmitted through the gascell 6. The light path “L” in this embodiment was 30 m. This light pathis variable, and the gas cell size 6 can be very small with multiplereflection of the light 12. After passing through and out of the cell 6,the light beam 12 was directed to and was received by the opticalspectrometer 8. The experimental data were collected and handled by alock-in amplifier and a computer 14. Nitrogen and target gases 16 wereloaded into the gas cell 6 in the same time but with different flowrates to obtain the different gas concentrations. According toBeer-Lambert law, when specific infrared radiation passes through thegas, the gas molecules absorb light energy:I=I ₀ e ^(−KCL)  (equation 1)where I₀ and I denote the input and output light intensities,respectively; K is the absorption coefficient of that gas; C is the gasconcentration; and L is the cell length. It is worth noting that thereis no filter needed in this system which is simpler, easier, and lessexpensive than current technology.

FIG. 11 shows the evolution of LED spectrum with the increasingconcentration of C₂H₂ in N₂. Compared with the absorption line of C₂H₂in FIG. 4, it is obvious that the intensity ranging from 1500 to 1560 nmdecreased because of the absorption of C₂H₂ and the intensity at 1525 nmproduces a maximal decline. The intensity lines on the graph at 1525 nmare, from the vertical highest to vertically lowest, 0, 100, 200, 300,and 400 ppm, the same order top to bottom as the values are listed inthe key in the top right of the graph.

The variations of LED spectra with the increasing concentration of CH₄in N₂ are shown in FIG. 12. The intensity lines on the graph atapproximately 1700 nm are, from the vertical highest to verticallylowest, 0, 1500, 3000, 4500, and 6000 ppm, the same order top to bottomas the values are listed in the key in the top right of the graph. Withthe increase of concentration, the intensity of the PL peak decreasesand this phenomenon conforms to Beer-Lambert law.

The same evolution was observed for NH₃ as shown in FIG. 13. The PLintensity at wavelengths of 1900 to 2060 nm dramatically decreases withthe increase of NH₃ concentration. The intensity lines on the graph at1950 nm are, from the vertical highest to vertically lowest, 0, 100,200, 300, and 400 ppm, the same order top to bottom as the values arelisted in the key in the top right of the graph.

When C₂H₂ with different concentrations was loaded into the gas cell,the area integral ranging from 1610 to 1840 nm and 1890 to 2070 nmcorresponding to CH₄ and NH₃ absorption were analyzed according to FIG.14. It is evident that the corresponding PL intensity was stable whichmeant the absorption of C₂H₂ barely affected the PL peaks relevant toCH₄ and NH₃. As shown in FIGS. 15 and 16, the same results were obtainedwhen CH₄ or NH₃ was loaded into the gas cell. The conclusion was thusobtained that there was little interference in the measurement of onegas while the other two gases are present.

By using the designed system, the concentrations of a series of preparedC₂H₂ samples between 0-800 ppm were measured (20° C., 101.325 kPa), FIG.17 shows the output signals at 15 standard concentrations of C₂H₂. Thecurve fitting was made using Matlab, and the fitting formula wasobtained as:y=1763×e ^(−x/318)+7667  (equation 2)where y is the area integral of the PL spectrum and x is the gasconcentration. Different concentrations of C₂H₂ were analyzed and thecalibrated formula was used to calculate the measured concentration tocheck the sensitivity and accuracy of the system. Comparing with thestandard concentration, the accuracy curve is shown in FIG. 17 inset.The sensitivity was 2×10⁻⁵ (20 ppm) and the accuracy was better than 2%[meaning the error rate was less than 2%]. The same calibratingexperiment and accuracy analysis were performed to obtain the calibratedformulas of CH₄ and NH₃. FIGS. 18 and 19 show the integral outputsignals of PL spectra for CH₄ and NH₃. Their respective calibratedformulas were determined to be:y=5670×e ^(−x/4633)+20563  (equation 3)andy=6474×e ^(−x/370)+17450  (equation 4).The detection sensitivities of CH₄ and NH₃ were 1×10⁻⁴ (100 ppm) and2×10⁻⁵ (20 ppm), respectively. Because of a smaller gas absorptioncoefficient, the sensitivity of CH₄ was lower than those of C₂H₂ andNH₃. The same accuracy of 2% was obtained according to the insets ofFIGS. 18 and 19.

Although a 3-gas simultaneous detection and measurement method and thedevice fabrication in NIR wavelength range are discussed and shown,based on the information disclosed, the device can be fabricated forsimultaneous detection and measurement of four, five, six or more gases,including ten or more. Further, the number of gasses detected can alsobe enhanced by incorporating other sized PbSe, PbS, or PbTe QDs, as wellas other NIR QDs and their core/shell variations. The disclosed processcan also utilize ultraviolet (UV) emission QDs such as ZnS, ZnSe (andother UV QDs and their core/shell variations) and visible (VIS) emissionQDs such as CdSe, CdTe (and other VIS QDs and their core/shellvariations), as well as QDs of CdS, CuInS₂, InP (and other QDs and theircore/shell variations that can emit light in more than one wavelengthranges, such as UV-VIS or VIS-NIR) for compact UV-VIS-NIR wavelengthlight sources for gas detection and measurement. This design can also beextended to IR range for QDs with good IR emission. In the Example, thetwo close PL peaks (in another word, wavelength) are 1515 nm and 1665nm. Practically the peaks/wavelengths can be closer to work on moregases in the same time with any computer peak analysis programs. Sometypical wavelengths of gasses that can be used for gas detection andmeasurement are: SO₂ (190-230 nm, 290-320 nm, 350-390 nm, 3.98 μm), CO(2.3 μm, 1.57 μm), CO₂ (2.7 μm, 4.33 μm), H₂S (1576 nm).

EXAMPLE

Five (5) mixtures of C₂H₂, CH₄ and NH₃ with different ratios were loadedinto the gas cell. FIG. 20 shows the corresponding PL, spectra with themixtures. The variation trend of the PL spectra of the mixed gasses wasthe same as the PL spectra with single gas. These examples also indicatethat the interference among the three gases is little. Based on theabove three fitting formulae, the method of cross calibration wasadopted to determine the measured concentration. FIGS. 21-23 show thecomparison between standard and measured concentrations for C₂H₂, CH₄and NH₃, in mixtures, and their accuracies were all better than 2%. Thisas-fabricated NIR QD-LED can detect C₂H₂, CH₄ and NH₃ simultaneously andwith the same high accuracy as when examined separately.

While various embodiments of the present invention have been describedin detail, it is apparent that various modifications and alterations ofthose embodiments will occur to and be readily apparent those skilled inthe art. However, it is to be expressly understood that suchmodifications and alterations are within the scope and spirit of thepresent invention, as set forth in the appended claims. Further, theinvention(s) described herein is capable of other embodiments and ofbeing practiced or of being carried out in various other related ways.In addition, it is to be understood that the phraseology and terminologyused herein is for the purpose of description and should not be regardedas limiting. The use of “including,” “comprising,” or “having” andvariations thereof herein is meant to encompass the items listedthereafter and equivalents thereof as well as additional items whileonly the terms “consisting of” and “consisting only of” are to beconstrued in the limitative sense.

I claim:
 1. A gas detection device comprising: a light emitting sourceincluding a first plurality of quantum dots of substantially discretesize and made of one or more semiconductor materials forming a firstlayer; and a second plurality of quantum dots of substantially discretesize and made of one or more semiconductor materials forming a secondlayer; a gas cell to contain the gas to be detected; and a lightdetector; wherein a quantum dot in any one layer is within 1.0 nm indiameter of size compared to a quantum dot in any adjacent layer.
 2. Thegas detection device of claim 1 wherein the cell includes a convexmirror arrangement to increase a light path through the gas cell tomultiple times a length of the gas cell.
 3. The gas detection device ofclaim 1 wherein the first plurality of quantum dots are one ofsubstantially different size, substantially different composition, andboth substantially different size and substantially differentcomposition compared to second plurality of quantum dots.
 4. The gasdetection device of claim 3 further comprising a third layer of quantumdots including a third plurality of quantum dots, and the thirdplurality of quantum dots are one of substantially different size,substantially different composition, and both substantially differentsize and substantially different composition compared to each of thefirst and the second plurality of quantum dots.
 5. The gas detectiondevice of claim 1 wherein the first plurality of quantum dots include atleast one of Pb, Se, S, Te, Zn, Cd, Cu, In, P, and a combinationthereof.
 6. The gas detection device of claim 1 wherein the firstplurality of quantum dots include at least one of PbSe, PbS, PbTe, ZnS,ZnSe, CdSe, CdTe, CdS, CuInS2, and InP.
 7. The gas detection device ofclaim 1 wherein the first plurality of quantum dots are between 4 nm and7 nm in size.
 8. The gas detection device of claim 1 wherein the firstplurality of quantum dots exhibit photoluminescence with a wavelengthemission in at least one of Infra-red, Near Infra-Red, visible, andultraviolet wavelengths of light.
 9. The gas detection device of claim 1wherein the light emitting source includes a multi-wavelength lightemitting diode.
 10. The gas detection device of claim 9 wherein themulti-wavelength light emitting diode does not exhibitmutual-absorption.
 11. The gas detection device of claim 1 wherein thefirst plurality of quantum dot includes a material with a quantum yieldof at least 85%.
 12. A method of detecting one or more gasses using alight emitting source having at least a first plurality of quantum dotsof substantially discrete size and made of one or more semiconductormaterials forming a first layer and a second plurality of quantum dotsof substantially discrete size and made of one or more semiconductormaterials forming a second layer, a gas cell to contain the gas to bedetected a light detector, and a quantum dot in any one layer is within1.0 nm in diameter of size compared to a quantum dot in any adjacentlayer, the method comprising the steps of: energizing the light emittingsource; emitting first light waves from the first plurality of quantumdots; and passing the first light waves through a gas sample in the gascell and into a light detector.
 13. The method of claim 12 furthercomprising the step of emitting second light waves from the secondplurality of quantum dots and passing the second light waves through thegas sample in the gas cell and into a light detector.
 14. The method ofclaim 13 wherein the light emitting source includes a third plurality ofsemiconductor quantum dots of substantially discrete size and furthercomprising the step of emitting third light waves from the thirdplurality of quantum dots, passing the third light waves through the gassample in the gas cell and into a light detector, and the first, second,and third light waves are emitted at substantially a same first time andthe first, second, and third light waves are detected at substantially asame second time.
 15. The method of claim 14 wherein first layer isadjacent to one of a blue or ultraviolet light emitting diode chip; thefirst plurality of quantum dots has a first wavelength photoluminescenceemission; the second layer is adjacent to the first layer and the secondplurality of quantum dots has a second wavelength photoluminescenceemission that is shorter than the first wavelength, and the third layeris adjacent to the second layer and spaced from the first layer by thesecond layer, and the third plurality of quantum dots has a thirdwavelength photoluminescence emission that is shorter than the first andthe second wavelength.
 16. The method of claim 13 further comprising thestep of detecting two or more gases.
 17. The method of claim 12 whereinthe gas sample contains one of a single gas, more than one gas, and atleast three gasses.
 18. The method of claim 12 further comprising thestep of detecting at least one gas.
 19. The method of claim 12 furthercomprising the step of reflecting the first light waves off of mirrorsand passing the first light waves through the gas sample more than oncebefore passing the first light waves into a light detector.
 20. A gasdetection device comprising: a non-mutually absorption multi-wavelengthlight emitting diode; the light emitting source including a firstplurality of quantum dots of substantially discrete size and made of asemiconductor material, a second plurality of quantum dots ofsubstantially discrete size and made of a semiconductor material, and athird plurality of quantum dots of substantially discrete size and madeof a semiconductor material; the first plurality of quantum dots are oneof substantially different size, substantially different composition,and both substantially different size and substantially differentcomposition compared to second and the third plurality of quantum dots;the second plurality of quantum dots are one of substantially differentsize, substantially different composition, and both substantiallydifferent size and substantially different composition compared to thirdplurality of quantum dots; the light emitting source including at leastthree layers, the first layer including the first plurality of quantumdots and not the second or third pluralities of quantum dots, the secondlayer including the second plurality of quantum dots and not the firstor third pluralities of quantum dots, and the third layer including thethird plurality of quantum dots and not the first or section pluralitiesof quantum dots; a gas cell to contain the gas to be detected; the cellincluding a convex mirror arrangement to increase a light path throughthe gas cell to multiple times a length of the gas cell; a lightdetector; one of the first, the second, and the third pluralities ofquantum dots includes at least one of PbSe, PbS, PbTe, ZnS, ZnSe, CdSe,CdTe, OdS, CuInS2, and InP, the first, the second, and the thirdpluralities of quantum dots are between 4 nm and 7 nm in size; and thefirst, the second, and the third pluralities of quantum dots eachexhibit photoluminescence with a wavelength emission in at least one ofInfra-red, Near Infra-Red, visible, and ultraviolet wavelengths offight, wherein a quantum dot having a largest diameter in the firstlayer has a diameter no more than 75% larger than a quantum dot having asmallest diameter in the first layer, a quantum dot having a largestdiameter in the second layer has a diameter no more than 75% larger thana quantum dot having a smallest diameter in the second layer, and aquantum dot having a largest diameter in the third layer has a diameterno more than 75% larger than a quantum dot having a smallest diameter inthe third layer, and wherein a quantum dot in any one layer is within1.0 nm in diameter of size compared to a quantum dot in any adjacentlayer.